Devices in the form of implants for stimulating living tissue are already known. For instance, implants for the retina of the human eye have been developed that are provided for the purpose of treating patients whose eyesight has been partially or completely lost as a result of defects in the retina. In this case, in principle a microelectronic device having a plurality of light-sensitive pixel elements is implanted in the region of the retina, via which an image projected onto the retina by the still-intact lens of the eye is registered; alternatively, the registration of the image may also be effected by means of an external camera. The image registered by the pixel elements or by the camera is converted into electrical signals and output to the surrounding tissue—or, to be more exact, to the cells of the retina—via stimulating electrodes by means of electrical stimulating pulses, in order in this way to restore or to improve the eyesight of the patient who has lost, or partially lost, his or her sight.
Imbalances of electric charges on the stimulating electrodes may arise in the course of the stimulation of living tissue or nerves by individual or repeated stimulating pulses via stimulating electrodes. In known stimulation systems or stimulators, use is frequently made of pulse-generators, in order to generate the electrical stimulating pulses at the stimulating electrodes. In this case the shape or contour of the electrical stimulating pulses is adapted to the type of tissue to be stimulated. Via a current-generator the stimulating electrodes have electric current applied to them that corresponds to the electrical stimulating pulses generated by the pulse-generator.
However, after an individual stimulating pulse a small electric charge may be left behind on the stimulating electrode, for example by reason of errors or tolerances. A continuously residual or increasing imbalance of the electric charges on the stimulating electrodes may result in an undesirable flow of current between the stimulating electrodes, and hence in damage both to the tissue and to the stimulating electrodes, which may lead to the destruction of the stimulating electrodes and to the total failure of the stimulation system. By reason of imbalances of electric charges on the stimulating electrodes, in particular a flow of direct current may occur between the stimulating electrodes, which for the patient may be associated with unpleasant sensations and disadvantageous effects on the tissue or on the nerves.
In some known stimulation systems the elimination of imbalances of electric charges on the stimulating electrodes is effected, for example, by short-circuiting the electrodes or by means of parallel resistors for passive discharging of the stimulating electrodes or of series capacitors which are customarily used. However, such devices have the disadvantage that they are associated with a large space requirement, for it is desirable, in principle, to accommodate stimulation systems in as small a space as possible.
U.S. Pat. No. 6,301,505 B1 describes a device for stimulating nerve tissue, in particular in the inner ear or pertaining to muscle tissue. An electrical circuit monitors the build-up of potential between the stimulating electrodes. As soon as a potential between the stimulating electrodes is detected that is too high, further stimulations are prevented, so that the potential difference between the stimulating electrodes is unable to build up further. A stimulation of the stimulating electrodes is then suppressed until such time as, by virtue of a short circuit between the electrodes, the potential difference has been equalised again or lies below the limiting value. This device has the disadvantage that no stimulation can be performed until a potential difference between the stimulating electrodes has been equalised.
DE 101 51 650 A1 describes an electrode arrangement for electrical stimulation with a stimulating electrode, via which a stimulus signal is supplied to biological material, and with a counter-electrode. In addition, the electrode arrangement is equipped with a sensor electrode, with which a polarising voltage on the stimulating electrode is determined, as a result of which static portions of the electrode polarisation can also be detected. According to this known process, the polarising potential is measured continuously, and the stimulating signal is influenced in such a manner that the polarising potential between the stimulating electrodes does not exceed a defined value. This is obtained either by adjusting the amplitude or by switching the stimulating signal off. The disadvantage of this electrode arrangement consists in the fact that with the sensor electrode an additional electrode is needed for measuring the polarising potential, which increases the costs of the stimulation device, the effort in connection with the implantation, and the damage to the tissue to be stimulated. Furthermore, with this known method the measurement of a potential difference between the stimulating electrodes is effected during the stimulation, as a result of which the result of measurement may be impaired.
The object underlying the present invention is to create a device having little space requirement for the purpose of controlling the electric charge on stimulating electrodes, said device reducing or eliminating an undesirable flow of current between the stimulating electrodes of a stimulation system by reason of imbalances of electric charges on the stimulating electrodes.
This object is achieved by means of the device according to the invention with the features according to Claim 1 and also by means of a process with the features according to Claim 14. Advantageous further developments of the invention are specified in each of the dependent claims.